I think that if the "poly" familial model was more mainstream, Unicorns would be much less rare. We are all blinded by what society tells us is right, so even when a poly relationship is mentioned, some might get defensive even though it might work for them at some point in time. I am, of course, referring to my own trials. I have only now discovered this as a viable possibility, even though I had a couple of friends in a poly relationship. I always thought, "they just say their open, but he is really just cheating." I could never imagine that it was a trusting and loving relationship.
So look for a girlfriend (it might help if she is bi) who is open minded. Even if she doesn't like the idea at first, it might grow on her in time as her view of "what is possible" starts to change. Always be upfront about yourself, but know when to divulge proper info. Good luck on your search!
(Hey no glum faces, this is an adventure!)